Various technologies are available for manufacturing grinding rods for use in grinding mills, such as in ore crushing, stone crushing and the like. Grinding rods are usually 3 to 6 meters in length depending upon the size of the grinding device and have diameters which usually range from 7 to 10 cm. It has been found that the useful life of a grinding rod may be improved if it has a hard outer shell usually of martensitic microstructure and relatively soft end portions which are substantially of pearlitic microstructure. The soft end portions minimize rod spalling and splitting thereof and reduce breakage and wear of the rod mill liners. A discussion of grinding rods having soft end portions may be found in U.S. Pat. No. 4,589,934 as well as the several other U.S. patents discussed in the background of that U.S. patent.
In an attempt to improve grinding rod longevity by way of heat treatment, the chemistry of the steel in the grinding rod may be modified such as described in U.S. Pat. No. 4,840,686. The modification of the chemistry in the steel of the grinding rod results in the rod core having a bainitic microstructure with less than 10% pearlite and a core hardness of at least about 40 Rockwell C, or 40 HRC. It is thought that making rods with the proper selection of molybdenum and chromium to provide a rod core of mostly bainite enhances the wear rate resistance of the rod by nearly 20% over that of a conventional heat treated rod. The selected chemistry and heat treatment ensures that the core is of the harder bainite where softer pearlitic material is to be avoided.
The rods, as made in accordance with either of U.S. Pat. Nos. 4,589,934 and 4,840,686 are quenched after heating by passing the rod through a quench spray. The quenching of the rod is commenced inwardly of the leading end of the rod and the quench spray turned off short of the trailing end of the rod. It is thought that by not applying quench water spray to the leading end and trailing end of the rod, softer end portions are developed. Also as taught, the rod may have to pass through multiple quench zones in order to achieve the desired extent of quenching to ensure the formation of the harder martensitic shell. As is described in U.S. Pat. No. 4,589,934, minor amounts of quench water travelling along the rod surface towards either the leading or trailing end portion may create a wash effect, thereby expediting cooling of the end portion resulting in the formation of end portions which can have a hardness greater than 30 and perhaps up to 45 or 50 HRC. To minimize this effect, the commencing of the quench water spray and terminating of the quench water spray are activated or deactivated a considerable distance from each end. A significant portion of the rod end is not treated resulting in a fairly large transition zone between the quench portion of the rod which has the martensitic structure and the untreated end portion of the rod which has the pearlitic structure. In practice, the softer end portions of the rod may extend upwards of 30 cm or more with a very gradual transition from the hard shell to the softer portion. This results in a grinding rod having a greater length of softer end portion with consequent increased wear.
A further problem with the grinding rod of U.S. Pat. No. 4,840,686 is that it was found in heavy duty grinding environments that the rods quickly broke up due to the abuse during grinding, even though it was thought that the harder bainitic core would resist such break up. It is a natural assumption that, in order to increase wearability of the rods in heavy duty grinding environments, the rods would naturally have a harder outer martensitic shell. However, neither grinding rods with bainitic cores or much softer pearlitic cores perform very well in heavy duty grinding environments. Such rods with the harder outer martensitic shells tend to break up too quickly thereby rendering them useless in the grinding environment. It is understood, of course, that such rods with the harder outer shells, particularly with pearlitic cores, function very well in light duty and medium duty grinding environments. There are no exacting criteria as to what constitutes light duty, medium duty and heavy duty grinding environments. Consideration is however given to the critical speed of the mill where it is understood that increasing critical speed increases the duty of the grinding environment. Other factors include the diameter of the mill, the diameter of the rods in the mill and the length of the rods where it is understood that the longer rods tend to break up more readily than the shorter rods. With these criteria in mind, one skilled in the art can predict the duty of the grinding environment and hence select rods appropriate to that end use.
Accordingly, this invention provides grinding rods which have the desired wearability and have the desired durability in heavy duty grinding environments. This advantage of this invention has been surprisingly provided by way of a stress relieving technique for tempered grinding rods, particularly those having deeper and harder martensitic shells of a hardness greater than 50 and usually 55 to 65 Rockwell C. Although stress relieving techniques have been used in conjunction with tool steels, this is generally understood by those skilled in the art to perform different functions in view of the high alloy contents and high carbon contents of tool steels. The purpose of stress relieving is to modify the structure of the tool steel so that stress relieving of tool steels is conducted at relatively high temperatures usually around 500.degree. C. In view of the high alloy content, it is generally understood that stress relieving at these high temperatures brings about a change in the characteristic of the tool steel. Conversely, it is generally understood that tempering of mild steel products after the first temper does not bring about any significant changes in the physical characteristics of the product.